Mosasaurs were aquatic reptiles that died off during the end of the Cretaceous period 66 million years ago. These humongous marine lizards resided in every ocean all over the world, and did not transfer inland to deliver offspring.

The debate on whether these creatures were cold-blooded or warm-blooded has been going on for a while. In 2010, a team of scientists had suggested that the ancient marine reptile species known as mosasaurs were cold-blooded creatures or ectotherms.

However, several members of the scientific community thought otherwise. Alberto Perez-Huerta, along with colleagues Celina Suarez and T. Lynn Harrell Jr., now published a paper that suggests mosasaurs were actually warm-blooded or endotherms and capable of controlling their body heat.

Perez-Huerta, an associate professor of geology at the University of Arkansas, says the 2010 paper reported the thermoregulation - how body heat is controlled - of iconic extinct species such as ichthyosaurs, mosasaurs and plesiosaurs.

He says the conclusion of the 2010 study bothered him a bit because the report did not use a warm-blooded species for comparison.

"We know that size can matter in terms of thermoregulation," he says.

In the new study, which was led by Harrell, the trio used an oxygen isotope analysis on the fossils of mosasaurs from the collection of the university's Alabama Museum of National History.

The trio compared the mosasaur fossils to that of cold-blooded animals including turtles and fish from the same period, as well as that of warm-blooded creatures represented by birds, which are true endotherms.

Harrell had come up with good ideas for two chapters in his dissertation, which had also been published, Perez-Huerta says. The three of them then discussed looking for endothermy in mosasaurs, given Harrell's extensive knowledge on these extinct aquatic reptiles, the collection of fossils in the museum and the controversy linked to the Science paper.

In the end, the researchers found that compared to the body temperature of modern, endothermic birds, the body temperature of mosasaurs were similar, suggesting that the ancient marine reptiles were indeed warm-blooded.

This tendency toward higher body temperature remained despite the size of the genus or species. Gigantothermy, or the body size, did not matter.

The findings of the new study are featured in the journal Palaeontology.